1. Field of the Invention
The field of the invention is digital controllers, and more particularly, programmable controllers used to monitor and direct the operation of industrial machines and processes.
2. Description of the Prior Art
Programmable controllers are being used in a growing variety of industrial applications. Controllers such as disclosed in Dummermuth, U.S. Pat. No. 3,942,158, issued Mar. 2, 1976, have been used to perform large control tasks, such as supervising an entire assembly line or industrial process from beginning to end.
Now that programmable controllers have been proven in such large applications, industry is converting from old technology electromechanical controls to the new programmable controls for many smaller operations. In an allowed application of Dummermuth et al, U.S. Pat. No. 4,165,534, issued Aug. 21, 1979, a smaller controller with a microprocessor-based processor unit is disclosed. This controller has recently been retrofitted with an I/O interface rack of the type disclosed in Struger et al, U.S. Pat. No. 4,151,580, issued Apr. 24, 1979, which is less expensive and less elaborate than the rack of the controller in U.S. Pat. No. 3,942,158. This controller now provides a physically smaller, less expensive option of the first controller discussed above, while still providing the control capacity for a large number of industrial tasks.
There are still applications, however, where a smaller controller could be used. Some potential users, however, might be reluctant to invest in small controllers, where a growth in production demand and capacity could render such small controllers unusable or obsolete. Therefore, programmable controllers must become part of a controller system that provides the full range of controller capabilities through a set of compatible processors and I/O interface equipment of varying capacities.